Aug 2, 2015
Comets: From foretelling death to bringing life

Sen—Comets used to be regarded with fear and awe. As a child, I read that the ancients considered them harbingers of doom, which sent me running to the dictionary to find out what a harbinger was.

This week, we got confirmation that far from foreshadowing death and disasters, these celestial nomads may very well have been key to bringing life to Earth—and very possibly seeding it elsewhere in the Universe too.

It turns out that the comet is covered with organic compounds, including many never detected on a comet before, but which are found in the recipe of ingredients that cooks up a living cell. It seems we can still regard comets with awe, though for the amazing science they are revealing to us rather than for superstitious reasons.

The vast majority of comets to visit our skies are too faint to be seen without optical aid. Rosetta’s comet, though now famous thanks to the wonderful close-up images, is only visible from Earth with quite a sizeable telescope. One or two a year might become bright enough to be glimpsed in binoculars or with the unaided eye.

But now and again, a comet will come along that shines brightly in the night sky, looking unlike any other celestial phenomenon with one or more long tails stretching across the heavens. In the last 20 years or so, stand-out examples included Comets Hyakutake (C/1996 B2) and Hale-Bopp (C/1995 O1) in 1996 and 1997, and Comet McNaught (C2006/P1).

I was lucky enough to see all three, though the view of McNaught in an evening twilight sky over Richmond, south-west London, cannot compare to the magnificent spectacle it provided for southern hemisphere observers a couple of weeks later.

Hyakutake was a bright spring-onion/scallion comet that passed close to Earth, and I remember noting from my observing site in the south of France how its tail length grew noticeably from night to night as it approached us. It remains the only comet I have seen reflected in a puddle of rainwater! And no one who saw Hale-Bopp will forget what must have been the outstanding such visitor of the 20th Century. After photographing it over Stonehenge on the Easter weekend of 1997, I recall being constantly distracted by its brightness as I drove home.

Hale-Bopp was so obvious that I recall hearing a couple of neighbours discussing it over their front wall in a brightly lit London suburb. But generally, even with a bright comet, you need to get out under a dark sky, away from streetlights, to see it properly, including that delicate tail, with its fine structure shaped by the solar wind.

Imagine then how it must have seemed to ancient humans, untroubled by such concepts as light pollution, when a similarly brilliant celestial visitor arrived out of nowhere to disrupt the orderly workings of the heavens. They would have been used to the annual procession of familiar constellations of stars across the heavenly stage, and the movements of the Moon and naked-eye planets along a narrow band around the ecliptic. But comets were unexpected interlopers, unpredicted and unpredictable. It was thought they had to be there for a reason, and that reason was bound to be bad.

The earliest recorded sightings of comets were made by Chinese court astrologers, who believed that you could predict the nature of the omen from the nature and shape of the tails. Four tails meant disease, three a calamity affecting the state, while two curving tails were a prelude to war. Similar beliefs persisted around the world. Two thousand years ago, Roman philosopher Pliny the Elder also believed that the tail’s shape indicated the type of impending disaster, and the famous embroidery known as the Bayeux Tapestry shows an early depiction of Halley’s Comet while England’s King Harold is told of its appearance just a short time before the Battle of Hastings that saw his death.

More than 300 years BC, Greek scientist Aristotle believed that comets were a phenomenon caused by friction in our own atmosphere as the Earth rotated. Though others imagined a more cosmic origin, it was not until 1577 when eccentric Danish nobleman Tycho Brahe, a meticulous recorder of the positions of objects in the heavens in the days before the telescope, compared observations of a particularly bright comet, made across Europe, and determined that it must lie at least four times further away than the Moon.

The brightest comets have only been recorded once because they have come in from the Oort Cloud of icy objects, far beyond the outer Solar System, and so will not return, if at all, for many thousands of years. But other comets which must have originated in that remote zone are now regular visitors with orbital periods of just a few years, their paths having been altered by the gravitational influence of Jupiter. Rosetta’s comet is a fine example, with a period now of just 6.44 years.

English astronomer Edmond Halley first realised that a comet seen regularly throughout history was one and the same object and he predicted correctly its return in 1758. This comet, with a period of a little under 76 years, last visited us in 1986 and will return in 2061. Whereas most comets are named after their discoverer, Halley’s Comet bears his name for making this significant finding.

Since then, our understanding of comets has advanced using scientific reason rather than superstition, but bright comets still bring out the crackpots and charlatans, even today. When Halley’s Comet returned in 1910, unscrupulous opportunists were ready with “comet pills” to protect the gullible against poisonous gases in its tail. More recent sightings of such bright visitors have been accompanied by predictions of the end of the world. Tragically, Hale-Bopp’s appearance led 39 members of a cult in California to commit mass suicide, believing that an imaginary accompanying spaceship would carry their souls to a better place.

While Philae had to travel many millions of kilometers around the Solar System to study some comet dust, this month offers a good opportunity to see some that has come to us. August is one of the best months for observing meteors, those bright streaks caused by grains shed by comets as they vaporise in the Earth’s atmosphere. The strongest shower, called the Perseids, peaks on the night of Aug. 12/13, though meteors from the shower may be spotted for several nights either side of that date. With the Moon absent, skies will be dark, so why not check out some cometary debris for yourself!